This invention relates to mercury vapor discharge lamps and more particularly to fluorescent lamps. Still more particularly it relates to lamps that can be landfilled without leaching potentially damaging mercury into the environment.
Fluorescent lamps contain elemental mercury. During lamp operation, chemical reactions take place that convert some of the elemental mercury to salts or compounds, such as mercuric oxide (HgO), that are water soluble. There is a growing concern that a waste stream resulting from the disposal of fluorescent lamps may leach excessive amounts of this soluble form of mercury (Hg) into the environment. An acceptable method of measuring the amount of soluble mercury which may leach from the waste stream resulting from the disposal of fluorescent lamps is described in the Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987-26998 of volume 55, number 126 of the Jun. 29, 1990 issue of the Federal Register. The lamp to be tested is pulverized into granules having a surface area per gram of materials equal to or greater than 3.1 cm2 or having a particle size smaller than 1 cm in its narrowest dimension. The granules are then subject to a sodium acetate buffer solution having a pH of approximately 4.9 and a weight twenty times that of the granules. The buffer solution is then extracted, and the concentration of mercury is measured. At the present time, the United States Environmental Protection Agency (EPA) defines a maximum concentration level for mercury to be 0.2 milligram of leachable mercury per liter of leachate fluid when the TCLP is applied. According to the present standards, a fluorescent lamp is considered nonhazardous (and thus available to be conventionally land-filed) when less than 0.2 milligram per liter of leachable mercury results using the TCLP. Lamps that have leachable mercury concentrations above the allowable limit must be especially disposed of through licensed disposal operations. Disposal operators charge a fee for disposal of lamps that are not within the EPA""s limits. Therefore, customers must pay extra costs to dispose of these lamps. Customers of fluorescent lamps generally desire not to contend with disposal issues regarding mercury levels, and therefore some customers specify only those lamps which pass the TCLP standard.
Heretofore, efforts have been made to reduce the leaching of soluble mercury from fluorescent lamps during the TCLP testing as well as in landfills. Various methods have been proposed which attempt to treat or process burned-out discharge lamps or scrap lamp exhaust tubing containing mercury in order to reclaim the mercury and thereby reduce the amount of mercury-contaminated scrap.
U.S. Pat. No. 5,998,927, Foust, et al., teaches a method for inhibiting the formation of leachable mercury associated with a mercury arc vapor discharge lamp when the mercury is in elemental form The method comprises providing high-iron content metal components in the lamps, at least one of the high-iron content metal components having an amount of oxidizable iron of at least about 1 gm per kilogram of lamp weight.
What is not specifically addressed in the patent, however, is the situation in which practically all of the mercury may already be present in the soluble ionic form at the start of the TCLP testing, as a result of naturally occurring processes that take place within the fluorescent lamp during its operation.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the disposal of fluorescent lamps.
It is yet another object of the invention to allow conventional landfill disposal of fluorescent lamps of all diameters when the mercury contained therein is in the ionic form.
These objects are achieved, in one aspect of the invention, by the provision of a method for inhibiting mercury leaching from a mercury vapor discharge lamp wherein at least a part of said mercury is present as ionic mercury, and wherein the lamp includes an amount of oxidizable iron, the amount of iron being at least 1 gram per kilogram of lamp weight, and further depositing upon the inside surface of the glass lamp envelope a coating of tin oxide which is substantially free of a substance which would cause electrical conductivity of the coating. By xe2x80x9csubstantially freexe2x80x9d is meant that the concentrations of substances that would cause the coating to become electrically conductive are approximately at or below normal impurity levels.
The addition of the tin oxide provides a totally unexpected, synergistic effect between the tin oxide and the oxidizable iron to inhibit mercury leaching when the mercury is present in an ionic form.